Anti-falsification paper called "thread-inserted paper" is well known, in which an elongate element is inserted in the thickness of the paper. A typical example of the elongate element is a thread described hereinbelow. Paper with inserted threads requires very sophisticated technologies to manufacture and thus has a significant effect for anti-falsification. It is therefore widely used for making bank notes in many countries.
Thread-inserted paper may be classified largely into two categories. The first is the paper which has an elongate element embedded therein so that it is not exposed to the surface of the paper. The second is the one called "thread-inserted, window paper," in which a part of the inserted elongate element is exposed to the surface of the paper.
A variety of methods have been proposed for making the first category of paper. They include a method disclosed in Japanese Patent Laid-Open Specification No. 51-130309/1976, in which a nozzle is laid in the flow of paper stock in a slice portion of a Fourdrinier paper machine and an elongate element is supplied through the nozzle along with running water so as to be embedded in a web deposited on a wire cloth. A method is disclosed in Japanese Patent Laid-Open Specification No. 2-169790/1990, in which an elongate element incorporating device is used to incorporate an elongate element into the paper stock flowing from a flow box of a Fourdrinier paper machine by supplying the elongate element with an air flow that keeps the elongate element and the stock out of contact with each other. Also a method is disclosed in Japanese Patent Publication No. 5-40080/1993, in which a cylinder paper machine having two or more vats is used to combine two or more paper layers and an elongate element is fed from a pipe having projections and recesses in its inner wall to be inserted between the paper layers.
As the methods of making the second category of paper, the "thread-inserted, window paper", there is proposed a method disclosed in Japanese Patent Publication No. 5-85680/1993, in which a grooved belt mechanism with an elongate element passed through a raised end of a guide having recessed and raised portions is immersed in a suspension of paper stock on wire cloth. A method is disclosed in U.S. Pat. No. 4,462,866, in which a wire mesh having raised portions is used for a face wire of a cylinder paper machine and an elongate element is incorporated in the thickness of paper as the elongate element is fed in contact with the raised portions of the wire mesh so that the elongate element is incorporated in paper and exposed at window portions. Also, a method is disclosed in Japanese Patent Specification Laid-Open No. 6-272200/1994, in which a compressed air nozzle is installed in a rotating drum of wire cloth in a Fourdrinier paper machine to intermittently blow away pulp slurry deposited over an elongate element embedded in web so that the elongate element is exposed.
"Watermarked paper" having watermarks of letters or pictures also requires a high level of technology to manufacture and has long been used for anti-falsification. Because the anti-falsification effectiveness increases with the number of anti-falsification means employed in the anti-falsification paper, a variety of anti-falsification paper has been developed which combine the above-mentioned "thread-inserted, window paper" technology with the "watermarking" technology. One such example is sterling notes used in England, which has thread-inserted windows at predetermined locations and a watermarked portrait at a different location.
This conventional paper has a common drawback that the paper cannot be rolled up in a great length. Because the thread normally has a thickness of several tens of micrometers and is continuously inserted or embedded at a particular position of the paper, when the paper is rolled up, the paper thickness at the location where the thread is inserted progressively increases until cockles are produced at that location. Our investigation has found that when a thread of about 15 .mu.m thick is continuously inserted in paper of 100 g/m.sup.2, rolling about 1000 meters of the paper results in cockles being produced at the thread inserted location. Once cockles are produced, the paper rolled up thereafter is not adequate for use. Whether the paper can be wound up in a roll has a great effect on productivity at a later process where the paper will be subjected to printing, since the paper must be bundle-finished if it cannot be rolled up.
In anti-falsification paper that uses the "thread-inserted, window paper" technology and the "watermarking" technology at different locations, printing is difficult to be performed at the "thread-inserted, window" portion and the "watermarked" portion. If they are to be printed, only very light colors can be used. This reduces the area on paper where decorative printing can be made, giving rise to limitations on the designs which can be printed.